The need often arises for monitoring transmissions on a communications link, for example in order to determine whether the link and its associated transmission equipment are working properly and to diagnose the problem if they are working improperly. Various arrangements exist in the prior art for enabling a craftsperson to do so in certain environments. For example, the twisted-wire pair subscriber loop of a conventional analog baseband telephony system could be directly monitored by a service technician simply by connecting a handset across the tip and ring leads of the loop. Higher-rate analog simplex transmission systems could theoretically be monitored to some extent visually via an oscilloscope connected across the leads of the transmission line, and simplex or duplex non-time-division-multiplexed analog systems could be monitored by a service technician aurally using demodulators analogous to radio receivers.
Digital transmission systems present challenges not encountered in the analog domain, however. Not only are communications encoded in digital form which must be decoded in order to make the communications recognizable to the craftsperson, but they also typically employ protocols whose informational units are transmitted interspersed with the communications. In a full-duplex system where transmissions are proceeding in both directions at the same time, this produces a jumble of electrical signals on the transmission link that is not readily decipherable by any known means of external (i.e., non-invasive) observation. An example of such a digital transmission system is the digital subscriber loop of modem ISDN telephony systems.
The prior art has provided service technicians with an ability to monitor communications in such duplex digital transmission systems through the use of protocol analyzers. The use of a protocol analyzer requires the technician either to connect the analyzer to the communication link at a termination, i.e., an end thereof, or to cut the communication link and connect the protocol analyzer in series with the two portions of the link. In the first instance, the protocol analyzer substitutes for the transmitter/receiver at one end of the link and turns the link into a simplex communication medium. In the second instance, the protocol analyzer receives the oppositely-directed simultaneous (duplex) transmissions separately, each on a different one of the two portions of the link. In either instance, the protocol analyzer may decode received communications in the same manner as would a conventional piece of equipment on the receiving end of the link.
The problems with these approaches are self evident. In the first instance, they limit the physical points at which the link may be monitored to only the two points of termination, e.g., at a customer's premises and at a telephony central office. This is impractical for many applications, e.g. field link-servicing and repair. In the second instance, the transmission link must be physically interrupted--cut--to allow connection of the monitoring equipment thereto. This causes both physical damage to the link and interruption of any communications being transmitted thereon. The physical damage is difficult and expensive to repair in a fault-free manner, i.e., in such a way that it will not be likely to cause problems in the future, and the interruption of communications degrades communication capabilities and makes in-service testing of communication links impossible or at least undesirable.
Hence, what is required is a non-invasive technique--one that does not cause physical interruptions and electrical (service) disruptions--for monitoring transmission links, such as full duplex digital ISDN telephony loops, substantially at any point thereof.